1. Introduction
This invention relates to removal of contaminants from organic compositions. More particularly, this invention relates to removal of multivalent metal,ions and organic contaminants from photoresist compositions.
2. Description of the Prior Art
Photoresists are light-sensitive compositions used for the formation of images in the manufacture of electronic devices. Photoresist coating compositions typically comprise a light-sensitive component and a polymer binder dissolved in a solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986; 5,212,046; 5,216,111; and 5,238,776, each incorporated by reference for disclosure of photoresist compositions, processing, and use.
In the manufacture of high resolution integrated circuits, it is known that many processing liquids come into contact with a bare wafer or a resist coated surface. These include the resist itself and treatment chemicals such as organic liquids and aqueous solutions which contain acids, bases, oxidants, and other proprietary ingredients. At least 15 to 50 liquids of various compositions are used to clean wafers, prime surfaces, deposit resists or other polymers, develop, rinse, etch, and strip the resist. It is known that these solutions are the source of contaminants that can interfere with the performance of the integrated circuit. Thus, the reduction or removal of insoluble and soluble contaminants from processing fluids used for the production of integrated circuits before or during use is basic insurance for prevention of damage to the integrated circuit.
Photoresist liquids are known to contain particulate and ionic contaminants. For example, it is known that solid gels or insolubles form in photoresists due to dark reactions. In addition, soluble impurities such as moisture, silicone oils, plasticizers, and metal ions may be present from the manufacture of the resist components and from the packaging containers or dispensing tanks. Trapped bubbles from point-of-use filtration or the shaking of a resist bottle prior to dispensing can lead to defects in resist coatings. In Class 100 clean rooms, airborne particulate counts amount to 3 particles per liter of density of 2. By comparison, liquids contain about 100,000 particles per liter. A particle count of 100,000 per liter seems high, but if translated into a solid of 0.6.mu. in size (entity of 2), this is equivalent to 10 parts per million (ppm). A level of 10 ppm amounts to the deposition of 1 mg per liter. Since liquids are heavily contaminated compared to clean room air, effective contaminant removal is essential to the manufacture of such devices.
Ultrafiltration of resist liquids has progressed and manufacturers of resist now supply resist materials filtered through 0.04 .mu.M-diameter absolute filters. Other methods for removal of particulates such as gels include ultracentrifugation, electrostatic treatment of the resist, and depth filtration. These methods are useful for the removal of particulates but are not effective in removing dissolved contaminants such as organic impurities and ionic species. These contaminants can be as damaging to an integrated circuit as particulate contamination.
Dissolved contaminants in resists such as metal ions, organic contaminants and halide ions require more sophisticated detection and removal methods than the methods used to remove particulates. One :such method is disclosed in International Publication No. WO 93/12152 which is directed to removing metal ions such as sodium and iron from novolak resins during manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions within the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin. This method of removal is cumbersone, does not remove contaminants from other photoresist components and does not remove the ionic species from the photoresist that enters between the time of manufacture of the phenolic and the use of the photoresist.
Another process for removing metal ions from photoresist compositions comprises treating an ethyl lactate solvent with a cation exchange resin at elevated temperatures. This process is disclosed in U.S. Pat. No. 5,234,789, incorporated herein by reference. It is stated in the patent that the purification of the ethyl lactate solvent at elevated temperature in the presence of the cation exchange resin results in transesterification and saponification with the formation of a lactide and other by-products. In accordance with said patent, the presence of the lactide in the photoresist composition is said to be advantageous as the lactide is said to function as a lithographic speed enhancer. However, in practice, it has been found that the presence of the lactide makes the photospeed unpredictable and the process does not remove multivalent metal ions because the multivalent ions are often chelated with other impurities in the solution.
An additional process for removing metal ions from novolak resins is disclosed in U.S. Pat. No. 5,073,622, incorporated herein by reference. In accordance with this patent, a conventional novolak resin is dissolved in an organic solvent or solvent mixture in a concentration of from about 25 to 50 percent by weight and the resultant solution is contacted at least once with an acidic, preferably complex forming, compound. The contacting step is preferably performed by carrying out a liquid-liquid extraction which may include a single-stage or multi-stage crossflow or multi-stage countercurrent treatment. In accordance with the patent, metal ions contained in the novolak resin are complexed and extracted into an aqueous phase upon contact of the organic and aqueous phases with each other.